Tantalum carbide, for example, TaC has the highest melting point among transition metal carbides and high chemical stability. FIG. 10 shows a phase diagram of TaC. The application of the TaC has been conventionally sought for various applications under a high temperature atmosphere, and manufacturing methods due to various methods have been reported.
Examples of conventional methods for manufacturing TaC include the following.    Patent Document 1: Japanese Published Unexamined Patent Application No. 6-87656    Patent Document 2: Japanese Published Unexamined Patent Application No. 2000-44222    Patent Document 3: Japanese Published Unexamined Patent Application No. 8-64110    Patent Document 4: Japanese Published Unexamined Patent Application No. 7-330351    Patent Document 5: Japanese Published Unexamined Patent Application No. 10-245285    Patent Document 6: Japanese Published Unexamined Patent Application No. 2000-265274    Patent Document 7: Japanese Published Unexamined Patent Application No. 11-116399    Patent Document 8: U.S. Pat. No. 5,383,981
For example, the Patent Document 1 describes the following method. TaC powder of fine powder and fine powder of other compounds such as HfC, ZrC and HfN are mixed. The mixture is sintered at 2000° C. in a vacuum of approximately 1 Pa to form a solid solution of TaC and other compounds. A fine TaC sintered body is produced by controlling the grain growth of TaC.
The Patent Document 2 describes the following method. Tantalum oxide (Ta2O5) and carbon are mixed, and a primary carbonization is performed at a prescribed temperature in a hydrogen furnace. The amount of free carbon of the obtained carbide is measured. The amount of carbon is then adjusted based on the measurement result, and the carbon is added to a primary carbide. A secondary carbonization is then performed at a prescribed temperature in a vacuum carbonization furnace to manufacture TaC.
The Patent Document 3 describes the following method. Metal Ta is evaporated in a vacuum, and C2H2 gas is simultaneously introduced. Both are reacted at a pressure/layer formation speed of 6.0×10−2 Pa·min/μm during vapor deposition by a reactant ion plating method to coat a TaC layer having a composition ratio of 1<C/Ta<1.2, excelling in a heat resistance, providing a radiation current stably even in a state of poor vacuum, and having a long life on the surface of an electron emitting material made of tungsten.
The Patent Document 4 describes a mold release layer coated on the surface of a metal mold used when a highly precise glass optical element such as a lens and a prism is press-molded. The mold release layer is one kind selected from (a) a ceramic material composed by 50 to 99 mol % of chromic oxide and 1 to 50 mol % of tantalum oxide, (b) a ceramic material composed by 50 to 99 mol % of chromium nitride and 1 to 50 mol % of tantalum nitride, (c) a ceramic material composed by 50 to 99 mol % of chromium carbide and 1 to 50 mol % of tantalum carbide.
The Patent Document 5 describes a carbon composite material for a reducing atmosphere furnace capable of exhibiting an excellent reduction gas reaction controlling effect even in a hot reduction gas atmosphere exceeding 1000° C., and capable of prolonging a product life significantly. The carbon composite material is used as the layer of the tantalum carbide formed on the surface of a graphite substrate by an arc ion plating (AIP) type reactive deposition method using metal tantalum and reactive gas.
The Patent Document 6 describes a method for forming a conductive Ta layer by a CVD method using a conductive Ta layer forming material containing a compound having Ta and a hydrocarbon solvent.
The Patent Document 7 describes the following method. A Ta substrate is arranged on the inner wall of a crucible made of graphite. The crucible is filled with carbon powder so as to come into contact with the Ta substrate to cover the Ta substrate. Then, the crucible made of graphite is heated to carbonize the Ta substrate, and TaC is coated on the inner wall of the crucible made of graphite.
The Patent Document 8 describes the following method. A carbon source is applied to the surface of Ta or Ta alloy in a vacuum furnace heated at 1300° C. to 1600° C. to form a TaC and Ta2C layer. A TaC is then formed by performing high temperature annealing heating in a vacuum so that unreacted carbon atoms adhered to the surface are diffused in the Ta substrate to perform a carbonization treatment.
However, since the TaC powder of fine powder and the fine powder of other compounds such as HfC, ZrC and HfN are mixed, and sintered at 2000° C. in a vacuum of approximately 1 Pa and to produce TaC, the Patent Document 1 has a problem that the formation of TaC having an optional shape is difficult.
Since Ta2O5 and C are mixed and TaC is formed by two carbonization treatments after molding, the Patent Document 2 has a problem that it is difficult to form TaC having a prescribed shape as in one of the above Patent Document 1.
Since the layer of TaC is formed on the outer circumferential surface of the tungsten filament and the interface with the substrate such as tungsten is inevitably formed, it is difficult to avoid the generation of cracks and exfoliation or the like of TaC in the Patent Document 3.
One described in the Patent Document 4 is formed as a layer on the surface of the substrate as in one described in the Patent Document 3, and it is difficult to avoid cracks and exfoliation or the like of the ceramic material or the like composed by 50 to 99 mol % of the chromic oxide formed on the surface and 1 to 50 mol % of the tantalum oxide as in the Patent Document 3.
Since one described in the Patent Document 5 is obtained by forming TaC on the surface of the graphite material as the substrate by the arc ion plating type reactive deposition method, the interface between the substrate and the TaC is clearly formed as in ones described in the Patent Documents 3 and 4, and it is difficult to avoid cracks and exfoliation or the like of TaC.
Since one described in the Patent Document 6 is also obtained by forming the conductive Ta layer using the CVD method, and the interface between the substrate and the conductive Ta layer is formed as well as ones described in the above Patent Documents 3 to 5, it is difficult to avoid cracks and exfoliation or the like of the conductive Ta layer by a thermal history or the like.
In the Patent Document 7, TaC is formed on the surface of Ta by directly contacting Ta with carbon powder and by heat-treating them. It is considered that the boundary of Ta and TaC appears clearly though there is no particular description in the description. Thereby, the TaC layer may be peeled off by the thermal history.
In the Patent Document 8, as shown in FIG. 5A to FIG. 5F of the description, the Ta2C layer also disappears by diffusing the unreacted carbon atom existing on the surface into the Ta substrate by high temperature annealing after the formation of a Ta2C and TaC layer, and the bulk crystal of TaC having approximately twice the thickness as one before the annealing is formed. The boundary between the Ta substrate and the TaC is clearly divided in the enlarged photograph observation. Thereby, it is considered that the delamination between the layers and the crack of the TaC layer are easily generated by the heat stress received repeatedly though there is no description in the description.
Even if the native oxide layer Ta2O5 of the surface of the Ta substrate is reacted with the carbon atoms at a low temperature of 1300° C. to 1600° C., the native oxide layer of Ta2O5 is chemically stable, the carbonization speed of Ta is low, and the diffusion depth of the carbon atoms is very shallow. Thereby, even if the carbon atoms are diffused and the TaC layer is grown by performing the vacuum heating annealing for tens of hours, a desired thickness is not obtained. Simultaneously, crystal grains grow greatly by heating for a long period of time to be formed in a bulk shape, and the boundary is also larger. It is considered that the boundary between the Ta substrate and TaC is clearly divided, and the delamination between the layers and the crack in the TaC layer are easily generated.